Is it possible to migrate my server from HDD to SSD?

Yes, that is possible. There are, however, a few things you should take into account:

Your server will be offline during migration. The time this will take will depend on the size of the disk.

There is a possibility your server will have to change to another network segment. This is due to the capacity in the current segment. Your IP address will change if this is the case. If your server is required to move, you will see a confirmation about this beforehand, when upgrading to SSD. You will have to change the IP configuration of the VPS yourself when not using DHCP.

The rDNS will change. You can change it yourself from the Tilaa dashboard.

If you would like to upgrade your server from HDD to SSD or vice versa you can use the upgrade button on the status page of your VPS in the dashboard.